Niobium phytate prepared from phytic acid and NbCl₅: a highly efficient and heterogeneous acid catalyst

Abstract

The preparation of functional catalysts using naturally-occurring building blocks is of great importance. In this work, we designed a functional heterogeneous acid catalyst, niobium phytate, using phytic acid, which could be obtained from the seeds and grains of plants, as the building block to react with NbCl₅. The prepared niobium phytate was characterized by XRD, FT-IR, XPS, SEM, TEM, NH₃-TPD and N₂ adsorption–desorption examinations. Niobium phytate showed very high activity for both cyanosilylation of carbonyl compounds and dehydration of carbohydrates. It was also found that niobium phytate had higher activity than commercial Nb₂O₅. In addition, niobium phytate could be easily recovered and reused without reducing the reaction activity considerably. Further study indicated that both higher acidity and lower crystallinity contributed significantly to the excellent catalytic performance of niobium phytate. The findings in this work provide insights into the design of new functional catalysts using naturally-occurring building blocks for various organic reactions.

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Introduction

With increasing attention being paid to the utilization and transformation of biomass,¹ the design of efficient catalysts based on naturally-occurring building blocks has become a hot topic in recent years.² A salient feature of naturally-occurring building blocks is their ability to provide more opportunities for designing novel catalysts with specific functions to catalyze various reactions.

Different naturally-occurring building blocks have been used to generate efficient catalysts. Among various naturally-occurring compounds, amino acids are the most used compounds for the preparation of bio-based catalysts. For example, amino acids supported on polystyrene could be used as efficient catalysts for chemical fixation of carbon dioxide.³ Cu^II-complexes derived from amino acids could catalyze the asymmetric oxidative coupling of 2-naphthol.⁴ Amino acid ionic liquids were successfully synthesized and showed eminent catalytic activities towards aldol reactions and the synthesis of styrene carbonate from styrene oxide and CO₂.⁵ Besides amino acids, chitosan could be converted into chitosan–Schiff base complexes for aerobic oxidation of cyclohexene.⁶ Recently, Guo and co-workers found that tannic acid (TA) could coordinate with Rh^III to form Rh^III–TA capsules through the chelating ability of phenolic structures, which showed excellent performance for the hydrogenation of quinolone.⁷ These developments in catalyst preparation from naturally-occurring building blocks provide a wider view for designing efficient catalysts, which are interesting and attractive.

Phytic acid, a major phosphorus reservoir in plants, can be obtained from the seeds and grains of plants and widely used as a chelating agent, food additive, and antioxidant.⁸ Because there are six phosphinic groups in its structure (Scheme S1†), phytic acid can be used as the source of phosphoric acid to coordinate with various metal ions to form a novel metal phosphonate. Metal phosphonates are very useful functional materials, which have gained much attention owing to their diverse potential applications in adsorption, catalysis, separation, energy storage, biology, ion exchange, and functional materials.⁹ In particular, metal phosphonates have recently emerged as a class of promising catalysts or catalyst-supports for a wide range of chemical reactions, including asymmetric hydrogenations,¹⁰ diverse oxidations,¹¹ polymerizations,¹² aldol and Knoevenagel condensations,¹³ and so on. Very recently, a porous zirconium–phytic acid hybrid has been prepared and used as an efficient catalyst for Meerwein–Ponndorf–Verley reductions.¹⁴

Inspired by these achievements, we designed a novel Nb-containing heterogeneous catalyst, niobium phytate, by using phytic acid as the building block. Niobium phytate was formed by the coordination between Nb⁵⁺ and the phosphate groups in phytic acid. As examples of applications, the synthesized niobium phytate was used as an effective heterogeneous acid-catalyst for both cyanosilylation of carbonyl compounds and dehydration of carbohydrates to form 5-hydroxymethylfurfural (HMF), which are typical acid-catalyzed reactions. The as-prepared niobium phytate showed high catalytic activity for the two reactions. In addition, niobium phytate can be easily recovered and reused without a considerable decrease in catalytic activity for both reactions. Phytic acid, as a building block, has great potential in the preparation of functional heterogeneous catalysts.

Results and discussion

Catalyst characterization

The synthetic procedure for niobium phytate is presented in detail in the Experimental section. The powder XRD pattern of the synthesized niobium phytate is shown in Fig. 1a. It could be seen that the catalyst had no X-ray crystal structure. Furthermore, the XRD pattern showed one broad diffraction peak, indicating that the obtained catalyst was amorphous. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that the catalyst was composed of particles with a size of about 60 nm (Fig. 1b and c).

Fig. 1 Powder XRD patterns (a), SEM image (b) and TEM image (c) of the prepared niobium phytate.

The Fourier transform-infrared (FT-IR) technique was also used to characterize the synthesized catalyst (Fig. 2). The bands at 3400 cm⁻¹ and 1630 cm⁻¹ can be associated with the surface-adsorbed molecular water.¹⁵ It could be seen that there is one band at 1020 cm⁻¹, suggesting the formation of Nb–O–P networks in the synthesized catalyst. An additional strong band was also found at 625 cm⁻¹, which was assigned to the Nb–O stretching mode.¹⁴ UV-vis analysis of niobium phytate and Nb₂O₅ revealed that they had similar spectra (Fig. S1†), which indicated that Nb⁵⁺ in niobium phytate had a similar coordination pattern to Nb₂O₅.

Fig. 2 FT-IR spectra of niobium phytate.

X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical state of the prepared catalyst (Fig. 3). As shown in Fig. 3a, the Nb 3d signal in niobium phytate was composed of two single peaks located at 207.5 eV for Nb 3d_5/2 and 210.2 eV for Nb 3d_3/2, which were characteristic of Nb⁵⁺.¹⁶ The binding energy of P 2p at 133.6 eV was characteristic of P⁵⁺ (Fig. 3b). These results suggested the presence of Nb⁵⁺ and P⁵⁺ in the prepared catalyst.

Fig. 3 XPS spectra of Nb 3d (a) and P 2p (b) in niobium phytate.

The textural parameters of the synthesized niobium phytate were investigated by the N₂ adsorption–desorption method after the sample was degassed at 100 °C for 24 hours. It can be seen that the N₂ adsorption–desorption isotherm of the catalyst is similar to the type IV mode, showing pore condensation with pronounced adsorption–desorption hysteresis (Fig. 4). The results in Fig. 4 indicate that the niobium phytate prepared was porous. The porosity of niobium phytate could result from the networks formed by the coordination between Nb⁵⁺ and phytic acid. The XRD pattern (Fig. 1a) indicated that niobium phytate was poorly ordered. Therefore, it can be deduced that there were many irregular connectivities in niobium phytate, which was consistent with the fact that the pore size distribution was relatively wide (Fig. 4). The average pore diameter, the Brunauer–Emmett–Teller (BET) surface area, and the pore volume calculated from the N₂ adsorption–desorption isotherm were 12.4 nm, 30.9 m² g⁻¹, and 0.11 cm³ g⁻¹, respectively.

Fig. 4 N₂ adsorption–desorption isotherm of the synthesized niobium phytate.

Examination of catalytic activity

The as-prepared niobium phytate may find a wide range of applications in different fields. It is very stable and is not soluble in water and common organic solvents, which are excellent characteristics of heterogeneous catalysts. Generally speaking, Nb-containing compounds could be used as acid catalysts for various organic reactions. As examples of applications, we used niobium phytate as a heterogeneous catalyst to catalyze both cyanosilylation of carbonyl compounds and dehydration of carbohydrates to form 5-hydroxymethylfurfural (HMF), which are both acid-catalyzed reactions.

Cyanosilylation of carbonyl compounds is a convenient reaction to synthesize cyanohydrins, which are important intermediates in the synthesis of fine chemicals and pharmaceuticals.¹⁷ The synthesized niobium phytate was used as a heterogeneous catalyst for the cyanosilylation reaction of carbonyl compounds, including various aldehydes and ketones, with trimethylsilyl cyanide (TMSCN) to produce cyanohydrins, and the results are given in Table 1. It is obvious that the reaction did not proceed without any catalyst (Table 1, entry 1). To our delight, the niobium phytate catalyst showed excellent performance for cyanosilylation reactions at room temperature. We also studied the activity of commercial Nb₂O₅ for the cyanosilylation of cyclohexanone (Table 1, entry 6), and the activity was found to be lower than the as-prepared niobium phytate (Table 1, entry 2). The higher activity of niobium phytate could be caused by its higher acidity and lower crystallinity, which will be discussed in detail in the following section. The reusability of the as-prepared niobium phytate for the cyanosilylation reaction of cyclohexanone was also studied. It was indicated that the catalyst could be reused at least five times without reducing the catalytic activity, as shown in Fig. 5. The niobium phytate recovered after being reused five times was characterized by FT-IR, XRD, XPS, SEM, and TEM. As shown in the patterns of FT-IR (Fig. 6a), XRD (Fig. 6b), and XPS (Fig. S2 and S3†), it can be seen that the recovered catalyst showed the characteristic peaks, which indicated that the structure of the recovered catalyst was not changed. Meanwhile, from the SEM (Fig. 6c) and TEM (Fig. 6d) images, it is obvious that the structure of the recovered catalyst did not collapse and the particles showed no obvious aggregation compared with the fresh catalyst. XPS examinations revealed that the concentrations of Nb, P, C and O on the surface of fresh and used niobium phytate were not nearly changed (Table S1†) after five catalytic recycles. Further, the results of thermogravimetric analysis in this work indicated that the decomposition temperature of the as-prepared catalyst was about 300 °C (Fig. S4†), which was much higher than the reaction temperature. The above examinations proved that niobium phytate was stable in the catalytic process and its properties were not changed noticeably after being used.

Table 1 Cyanosilylation reaction of carbonyl compounds using TMSCN^a

Entry	Reactant	Catalyst	Reaction time (min)	Yield^b (%)
a Reaction conditions: carbonyl compound, 2 mmol; TMSCN, 4 mmol; catalyst, 0.1 g; CH2Cl2, 4 g. b Yields were determined by GC using ethylbenzene as the internal standard. c The conversion of cyclohexanone was 18% and the selectivity was about 72% with some unidentified by-products. d The amorphous Nb2O5 was prepared by hydrolysis of niobium(V) ethoxide.
1		None	20	0
2		Niobium phytate	20	98
3		Niobium phytate	15	96
4		Niobium phytate	15	100
5		Niobium phytate	15	98
6		Commercial Nb2O5	30	16
7		Phytic acid	20	13^c
8		Amorphous Nb2O5^d	30	37

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Fig. 5 Reusability of niobium phytate for the cyanosilylation reaction of cyclohexanone. Reaction conditions: cyclohexanone, 20 mmol; TMSCN, 40 mmol; niobium phytate, 0.1 g; CH₂Cl₂, 4 g, reaction time, 15 min; reaction temperature, room temperature.

Fig. 6 The characterization of niobium phytate after being reused five times. FT-IR spectrum (a), XRD pattern (b), SEM image (c), and TEM image (d).

We also used niobium phytate as an acidic heterogeneous catalyst for the dehydration of carbohydrates to form HMF in the ionic liquid 1-butyl-3-methylimidazolium chloride ([Bmim]Cl), which is a very important reaction for biomass conversion.¹⁸ We conducted the reaction using the synthesized niobium phytate, NbCl₅, and commercial Nb₂O₅ as catalysts under the same reaction conditions, and the results are shown in Table 2. Although the dehydration of fructose could proceed without catalysts in [Bmim]Cl, the yield of HMF was very low (entry 1, Table 2), indicating that a catalyst was necessary to achieve high reaction activity. It was found that niobium phytate showed the highest HMF yield (87.5%) (entry 2, Table 2) with full conversion of fructose. In contrast, although the conversion of fructose was also 100% over NbCl₅, the yield of HMF was only 73.6% (entry 3, Table 2) due to the side reaction. When Nb₂O₅ was used as the catalyst, a longer reaction time (1.5 h) was needed to convert fructose completely (entry 4, Table 2). But, the yield of HMF was still lower than the result obtained over niobium phytate because a longer reaction time could cause side reactions of HMF. These results indicated that niobium phytate was the better catalyst for the dehydration of fructose, which was comparable with many results reported in the literature using heterogeneous catalysts in various solvents.¹⁷ Recycling experiments indicated that niobium phytate could also be reused five times for the dehydration of fructose with a slight reduction in catalytic efficiency. The slight decrease in catalytic activity may have resulted from the leaching of Nb (about 21 ppm) in the reaction mixtures due to the strong solvent power of [Bmim]Cl, which could be examined by the ICP method. Furthermore, niobium phytate could also be used to catalyze the dehydration of inulin and sucrose in [Bmim]Cl to produce HMF, and yields of 56.4% and 45.4% (entries 5 and 6, Table 2), respectively, could be obtained with a prolonged reaction time (3 h). Compared with fructose, inulin and sucrose only gave moderate HMF yields. There are two reasons for this phenomenon. Firstly, the reaction process of inulin and sucrose to produce HMF was a two-step reaction involving hydrolysis and dehydration, which could increase side reactions and thus lower the reaction yield. Secondly, there were glucose units in the structure of inulin and sucrose and niobium phytate showed no activity for glucose. After the reaction, glucose was still detected by HPLC with a refractive detector (Shimadzu RID-10A). Only fructose units could be converted into HMF over niobium phytate, and thus the HMF yield was moderate when inulin and sucrose were used as substrates.

Table 2 Dehydration of carbohydrates to produce HMF^a

Entry	Reactant	Catalyst	Time (h)	Conversion (%)	Yield^bc (%)
a Reaction conditions: reactant, 0.1 g; catalyst, 0.1 g; [Bmim]Cl, 1 g; reaction temperature, 100 °C. b The yields were determined by HPLC using an external standard method. c The data in parentheses are the error bars. d Niobium phytate was reused for the fifth time.
1	Fructose	None	1	18.4	12.7(±1.3)
2	Fructose	Niobium phytate	1	100	87.5(±1.9)
3	Fructose	NbCl5	1	100	73.6(±4.5)
4	Fructose	Nb2O5	1	90.5	76.8(±2.6)
			1.5	100	81.6(±1.7)
5	Fructose	Niobium phytate (5th)^d	1	97.8	74.3
6	Inulin	Niobium phytate	3	100	56.4(±2.9)
7	Sucrose	Niobium phytate	3	100	45.4(±3.5)

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Explanation of the higher catalytic activity of niobium phytate

As discussed above, niobium phytate showed higher activity for the two examined reactions than Nb₂O₅, which was induced by two possible reasons, including higher acidity and lower crystallinity. Firstly, we determined the local environment of the Nb species in niobium phytate and Nb₂O₅ by the XPS method of Nb 3d. As shown in Fig. 7a, the binding energy values of Nb in niobium phytate corresponding to Nb 3d_5/2 and Nb 3d_3/2 were 207.5 and 210.2 eV, which were higher than the results of Nb₂O₅ (207.2 and 209.9 eV). The higher binding energy of Nb 3d in niobium phytate caused by the formation of P–O–Nb bonds in its structure could result in a higher positive charge on Nb atoms, which could increase the acidity of Nb.¹⁹ Further, the acidity of niobium phytate and commercial Nb₂O₅ was examined by the NH₃-TPD method. As shown in Fig. 7b, niobium phytate showed a peak at about 220 °C, which was related to the weak acid sites (mainly resulted from Nb⁵⁺). However, commercial Nb₂O₅ showed no obvious peaks. The results in Fig. 7b indicated that the amount of NH₃ desorbed from niobium phytate was higher than commercial Nb₂O₅, which was consistent with their acidity. We also conducted a pyridine absorption FT-IR analysis to identify the kind and number of acid sites in niobium phytate and Nb₂O₅ (Fig. 7c). From the figure, we could see that the characteristic Lewis acid sites at about 1606 and 1445 cm⁻¹ were observed in the spectra of both niobium phytate and Nb₂O₅.²⁰ The number of Lewis acid sites in niobium phytate was about 196 μmol g⁻¹, which was higher than that in Nb₂O₅ (78 μmol g⁻¹). The band at about 1487 cm⁻¹ is normally attributed to a combination band associated with both Brønsted acid and Lewis acid sites.²¹ Meanwhile, a band at about 1540 cm⁻¹ existed in the spectra of niobium phytate, which was characteristic of a Brønsted acid site.²² This Brønsted acid site resulted from the small amount of P–OH species in the phosphate groups,²³ which did not coordinate with Nb⁵⁺. In contrast, Nb₂O₅ showed no characteristic peak of a Brønsted acid site. The Brønsted acid sites existing in niobium phytate could also promote the reactions. However, when phytic acid was used as the catalyst for the cyanosilylation reaction of cyclohexanone, the yield of the desired product was only 13% with a selectivity of about 72% (entry 7, Table 1). This result indicated that the Lewis center (Nb) was very crucial for the high catalytic activity of niobium phytate. Therefore, based on the results obtained from XPS, NH₃-TPD and pyridine absorption FT-IR examinations, it can be concluded that the acidity of niobium phytate is much stronger than that of commercial Nb₂O₅. Because the two examined reactions were both acid-catalyzed reactions, the higher acidity of niobium phytate was beneficial to increase the rate of the two reactions.

Fig. 7 XPS spectra of Nb 3d (a), NH₃-TPD spectra (b), and pyridine absorption FT-IR analysis (c) of niobium phytate and Nb₂O₅.

Lower crystallinity was another reason for the higher activity of niobium phytate. The synthesized niobium phytate and commercial Nb₂O₅ were characterized by XRD (Fig. 8). It is obvious that the synthesized niobium phytate was amorphous with a very low crystallinity. In contrast, commercial Nb₂O₅ showed better crystallinity. The lower crystallinity of niobium phytate could be favourable for the reactants to be in contact with the acid center (Nb) of the catalyst, and thus promote the reaction with a higher activity. It was found that an amorphous Nb₂O₅ (Fig. 8) prepared by hydrolysis of niobium(V) ethoxide, which had a similar acidity to commercial Nb₂O₅ (Fig. S5†), showed better catalytic activity (entry 8, Table 1) for the cyanosilylation reaction of cyclohexanone than commercial Nb₂O₅, which indicated that lower crystallinity was indeed beneficial to the reaction. In addition, as shown in Table S2,† niobium phytate had a higher BET surface area, larger pore diameter, and larger pore volume than commercial Nb₂O₅, which were also helpful to increase the diffusion of reactants to the activity center to allow the reactions to proceed. Lower crystallinity combined with higher acidity resulted in the higher catalytic activity of niobium phytate.

Fig. 8 XRD patterns of niobium phytate, commercial Nb₂O₅, and amorphous Nb₂O₅ prepared by hydrolysis of niobium(V) ethoxide.

Conclusions

In summary, a functional heterogeneous acid catalyst, niobium phytate, could be designed using phytic acid as the building block to react with NbCl₅. It was discovered that niobium phytate showed very high activity for both cyanosilylation of carbonyl compounds with TMSCN and dehydration of carbohydrates to produce HMF, which are both acid-catalyzed reactions. Niobium phytate could be easily recovered and reused at least five times without decreasing the reaction activity and selectivity. Further study indicated that niobium phytate had higher activity than commercial Nb₂O₅, which could have resulted from the higher acidity and lower crystallinity of niobium phytate. The findings in this work provide insights into the design of new functional catalysts derived from naturally-occurring building blocks for various organic reactions.

Experimental

Materials

Fructose (99%), sucrose (99%) and 1-hexaldehyde (>95%) were purchased from Alfa Aesar. Inulin (95%), Nb₂O₅ (99.5%), niobium(V) chloride (99+%), trimethylsilyl cyanide (TMSCN, 97%) and furfural (98%) were provided by J&K Scientific Ltd. N,N-Dimethylformamide (DMF), CH₂Cl₂, ethyl ether, ethanol, methanol and cyclohexanone were A. R. grade and obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. Phytic acid with C. P. grade was received from Sinopharm Chemical Reagent Beijing Co., Ltd. Here, we needed to point out that phytic acid should be obtained from seeds and grains of plants.⁸ However, in this work, we mainly focused on the preparation of functional catalysts using phytic acid as the building block and thus we purchased it from a chemical company. The ionic liquid [Bmim]Cl used in the experiment was purchased from Lanzhou Greenchem ILs, LICP, CAS, China (Lanzhou, China) with a purity of over 99.9%. [Bmim]Cl was dried at 80 °C under vacuum for 96 h before use.

Synthesis of niobium phytate

Niobium chloride (the source of Nb⁵⁺, 22 mmol) and phytic acid (the ligand, 10 mmol) were dissolved in DMF (400 mL) in a 1000 mL flask. Then, triethylamine (the deprotonation reagent, 150 mmol) was added into the solution dropwise for 3 h. After that, the mixture was first stirred for 5 h at room temperature and then aged under static conditions at 80 °C for 6 h. Finally, the white precipitate was separated by filtration and thoroughly washed with DMF to remove the unreacted NbCl₅ and phytic acid. Before the characterization and utilization, the sample was washed with ethanol at 90 °C through Soxhlet extraction for 72 h to remove DMF completely. After that, the synthesized niobium phytate was dried at 80 °C under vacuum for 12 h. The yield of niobium phytate was about 89% (7.85 g). ICP analysis showed that the Nb/P molar ratio in niobium phytate was about 1 : 3. Anal. found: 21.27% Nb, 23.59% P, 9.13% C. Calcd. for Nb₂P₆C₆O₂₄H₈ (835.77): 22.23% Nb, 22.23% P, 8.62% C.

Cyanosilylation of carbonyl compounds

In a typical experiment, carbonyl compound (2 mmol), TMSCN (4 mmol), CH₂Cl₂ (4 g) and catalyst (0.1 g) were charged into a 10 mL flask equipped with a magnetic stirrer. The reaction mixture was stirred at room temperature for a desired reaction time. After the reaction, the products were analyzed by gas chromatography (GC, Agilent 7890) using ethylbenzene as the internal standard, and identification of the products was done by GC-MS (Shimadzu QP2010). In the experiments to test the reusability of niobium phytate, the catalyst was recovered by centrifugation and washed using ethanol and ethyl ether. After drying under vacuum at 80 °C for 12 h, the catalyst was reused for the next run by adding new reactants and solvents.

Dehydration of carbohydrates to produce HMF

In a typical experiment, carbohydrate (0.1 g), catalyst (0.1 g) and [Bmim]Cl (1 g) were charged into a 5 mL flask equipped with a magnetic stirrer. The mixture was stirred at 100 °C for the desired time. Then the mixture was cooled to room temperature immediately. The amount of HMF was analyzed by HPLC using an external standard method with a Shimadzu LC-15C pump, a Shimadzu UV-vis SPD-15C detector at 282.0 nm, and a Supelcosil LC-18 5ìm column at 35 °C. Before being analyzed, the reaction mixture was diluted to 1000 mL. Methanol/water solution (50/50 V/V) was used as the mobile phase at a flow rate of 0.8 mL min⁻¹.

Acknowledgements

The authors thank the Beijing Forestry University Fundamental Research Funds for the Central Universities BLX2014-40 and the National Natural Science Foundation of China (21473252, 21503016) for the financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5cy01123j

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